Vacuum actuated valves have long been used in the automotive industry for controlling the flow of various gasses, including exhaust gas recirculation (EGR). EGR systems use the valves as a means for allowing exhaust gas from the exhaust manifold to flow into the intake manifold. Thus, the exhaust gas is recirculated through the engine. Controlling EGR flow is typically used as means for meeting regulated emissions and maximizing fuel economy. Poor EGR flow control can have detrimental effects, such as engine misfires, engine speed surging, and lost fuel economy. In particular, valve opening delay is one aspect of EGR control that has a large effect on the system performance. Opening delay is defined as the time from when an engine control system commands the valve to open and allow EGR flow to the time when the valve actually opens to allow flow. Opening delay results from the preload, or biasing force, commonly used to hold the valve shut. In situations where no EGR flow is required, the preload force holds the valve shut so that disturbances do not accidentally open the valve.
Valve control systems are known in which valve position feedback is used to control the amount of EGR flow. By measuring the position of the valve and using a predetermined map between valve position and flow area, an indication of EGR flow can be obtained. Comparing this estimated EGR flow based on a valve position measurement to a desired EGR flow, a feedback signal is created. The feedback signal is then used to control an actuator to maintain the desired EGR flow. When no EGR flow is required, the biasing force is used to hold the valve closed against disturbance forces caused by exhaust pressure pulsations or engine vibration shaking forces that might tend to open the valve. As previously mentioned, it is necessary to keep the valve closed in certain instances because undesired EGR flow can affect emissions, driveability, and fuel economy. Such a system is disclosed in the U.S. Pat. No. 4,662,604. In addition, many valve designs, such as those used for EGR, are known which use a soft seat or stop to create a soft landing valve seat, for example, to reduce noise and increase sealing.
The inventors herein have recognized numerous disadvantages with the above approaches. One disadvantage is the presence of an opening delay, which is cause by the biasing force used to prevent disturbance forces from opening the valve when no EGR flow is desired. For an actuator to open the valve and allow EGR flow, the actuator must first create a large enough force to cancel the biasing force. Then, any additional force created by the actuator can be used to open the valve. Because actuators cannot be built to instantaneously create a force, without considerable expense and complexity, there is always a finite time delay between an open command and actual valve opening. The root cause of this opening delay is the presence of the necessary biasing force.